/Engine/Source/Runtime/Engine/Classes/Engine/LevelStreaming.h
Return whether this level should be present in memory which in turn tells the streaming code to stream it in. Please note that a change in value from false to true only tells the streaming code that it needs to START streaming it in so the code needs to return true an appropriate amount of time before it is needed.
true if level should be loaded/ streamed in, false otherwise
virtual bool ShouldBeLoaded() const